Generally, a hydraulic excavator, a hydraulic crane, or the like as a construction machine is largely constituted by an automotive lower traveling structure, an upper revolving structure mounted swingably on the lower traveling structure, and a working mechanism provided on the front side of the upper revolving structure.
In this instance, the truck frame which constitutes a main body portion of the lower traveling structure consists of left and right side frames and a center frame disposed between the side frames. The center frame is largely constituted by a top plate and a bottom plate which are connected to the left and right side frames at an interval provided therebetween in the vertical direction, a connecting tubular body provided between intermediate portions in the left and right directions of the top plate and bottom plate to connect the two plates, and a cylindrical circle member secured to an upper surface side of the top plate. Further, a swing circle is mounted on an upper end side of the circle member by means of bolts or the like, and the upper revolving structure is arranged to be swingably supported on the truck frame through the swing circle.
Here, the circle member which is secured on top of the center frame of the truck frame is generally formed into a cylindrical shape by bending rolled steel for general structure into a round shape, and a lower end portion of the circle member is secured to an upper surface side of the top plate for constituting the center frame by means of a welding means. A plurality of bolt holes (female screw holes) are threadedly provided in the upper end portion of the circle member over its entire circumference, and the swing circle is arranged to be mounted on the upper end portion of the circle member by use of the bolts which are threadedly inserted into these bolt holes.
In this instance, the circle member in accordance with a prior art is formed as a cylindrical body with a uniform plate thickness in the vertical (axial) direction in which the plate thickness is set to be equal from the upper end portion where the plurality of bolt holes are threadedly provided to the lower end portion which is welded to the center frame (Patent Literature 1: Japanese Patent Laid-Open No. Hei 10-306466 A).
Meanwhile, as the circle member in accordance with another prior art, the circle member which is used in a large size hydraulic excavator for use in, for example, the open-pit mining of minerals is formed into a stepped cylindrical shape by a cylindrical portion and a flange portion expanding to radially outward from a lower end side of the cylindrical portion. Further, this circle member constituted by the cylindrical portion and the flange portion is so arranged that a boundary portion between the cylindrical portion and the flange portion is cut out over the entire circumference into a recessed curved shape to thereby minimize the concentration of stress at the boundary portion between the two portions (Patent Literature 2: Japanese Patent Laid-Open No. Hei 11-71785 A).